Gaseous fuelling systems have a unique phenomenon associated with them called venting. Liquid fuelling systems have a related but different phenomenon called evaporative emissions, but venting does not occur in liquid fuel systems. Venting can occur for a variety of reasons. A pressure relief valve for a cryogenic storage vessel storing a liquefied gaseous fuel can be activated when vapor pressure within the vessel rises above a safety limit. In applications that employ a pressure regulating apparatus to maintain a pressure bias between a liquid fuel and a gaseous fuel, the apparatus can vent gaseous fuel when liquid fuel pressure changes rapidly.
Double-walled, vacuum insulated cryogenic storage vessels store liquefied gaseous fuel at cryogenic temperatures at low pressures. For example, when liquefied gaseous fuel is first added to the storage vessel it can be at or around atmospheric pressure and the temperature remains in equilibrium around the boiling point. However, since the storage vessel cannot reduce heat leak into the vessel completely, especially when a cryogenic pump is partially installed in the vessel, the liquefied gaseous fuel begins to boil, increasing the vapor pressure. A pressure relief valve in the vessel is employed to prevent excessive pressure build-up by venting gaseous fuel vapor. Since these cryogenic storage vessels are not specifically designed as pressurized storage vessels, the pressure at which these vessels vent can be relatively low, for example, around 500 pounds per square inch (psi).
In applications that employ concentric needle fuel injectors to introduce a gaseous fuel separately and independently from a liquid fuel, such as in the Applicant's co-owned U.S. Pat. No. 6,336,598, gaseous fuel pressure is maintained within a predetermined margin of liquid fuel pressure. Liquid fuel is employed not only as a pilot fuel but also as a hydraulic fluid and a fluid seal within the fuel injector. If gaseous fuel pressure rises above liquid fuel pressure (or if the pressure differential is below a predetermined level) then gaseous fuel leaks into the liquid fuel, and if gaseous fuel pressure drops too far below liquid fuel pressure, liquid fuel leaks excessively into the nozzle of the fuel injector and later gets burned in the combustion chamber. One way to regulate gaseous fuel pressure based on liquid fuel pressure is with a dome loaded regulating valve that employs the difference between liquid fuel pressure and upstream gaseous fuel pressure to modulate a valve such that downstream gaseous fuel pressure is maintained with the predetermined margin of the liquid fuel pressure. In these types of device, when the liquid-gaseous fuel pressure differential drops below the predetermined level, gaseous fuel is vented from the device such that the downstream gaseous fuel pressure can follow the rate of change of liquid fuel pressure thereby maintaining the pressure bias between the two fuels.
In addition to venting, gaseous fuelling systems have unique sealing challenges compared to liquid fuelling systems. Gaseous fuels have higher enthalpy since they are in a gas state compared to liquid fuels and so can penetrate through tighter spaces and further into solids. The dimensional tolerances between gaseous fuelling system components are smaller to reduce, and preferably prevent, the leakage of gaseous fuel. Seals employed between components should have a lower permeability than that required for seals used in liquid fuelling systems, and must be able to withstand harsh environments such as rapid decompression. Leakage of gaseous fuels after engine shutdown has been a particularly challenging sealing problem. After engine shutdown, gaseous fuel pressure remains high causing gaseous fuel to leak through fuel injectors.
During shutdown, liquid fuels can be depressurized by returning the fuel to the storage tank. Although not all the liquid fuel in fuelling conduits can be drained, by returning liquid fuel to atmospheric pressure the likelihood of leaks is substantially reduced. This is not possible in gaseous fuelling systems. For example, when gaseous fuel is stored as a gas in a pressurized storage vessel it is stored at a substantially higher pressure than the pressure required by the fuel injection system. The pressure of the gaseous fuel is reduced and regulated to the fuel injection pressure, and during shutdown cannot be returned to the storage vessel unless it is pumped by a compressor which is not economically feasible or efficient. When gaseous fuel is stored in liquefied form it is stored at cryogenic temperatures. As the engine demands fuel from the fuelling system, a cryogenic pump pressurizes and delivers the liquefied fuel from the cryogenic storage vessel to a vaporizer where it is converted to a gas, which is then supplied to fuel injectors for introduction into engine cylinders. During shutdown, if the vaporized gaseous fuel was returned to the cryogenic storage vessel it would add a considerable amount of heat to the vessel, causing the liquefied fuel to boil at a greater rate raising the pressure within the vessel leading to venting. As a result, after shutdown gaseous fuel remains in fuelling conduits and fuel injectors, and depending upon the fuel injection pressure the pressure of the gaseous fuel can be around one, two, or more, orders of magnitude above atmospheric pressure. The pressure differential between gaseous fuel pressure and atmospheric pressure after shutdown causes gaseous fuel to leak past injection valves in fuel injectors into engine cylinders, which can then eventually leak to atmosphere or result in unburned hydrocarbon emissions when the engine is started up again.
European Patent Specification No. EP 0 745 499, published on Jul. 11, 2001 to Michael Ernest Garrett, discloses a supplementary storage vessel fluidly connected with a vent pipe for receiving at least a portion of vented boil-off gas from a storage tank storing liquefied gaseous fuel. The supplementary storage vessel is filled with an adsorbent such as a high surface area activated carbon or zeolite sieve which has the ability to store a large volume of vented boil-off gas by adsorption. An auxiliary supply pipe is fluidly connected with the supplementary storage vessel for delivering the vented boil-off gas to an engine supply pipe.
International Patent Publication No. WO2012072184, published for Klaus Rossler on Jun. 7, 2012, discloses an arrangement for a combustion engine operated with gaseous fuel, where a supplementary storage unit is provided for receiving gaseous fuel from a fuel introducing device (e.g. a fuel rail for a direct injection system) during shutdown, such that leakage of gaseous fuel is avoided. A solenoid valve is employed to introduce the gaseous fuel into the intake manifold downstream of the throttle and compressor unit from a supercharger.
A problem with the techniques of the '499 and '184 references is that the gaseous fuel introduced from the supplementary storage tank/unit into the intake manifold does not burn well in the engine and substantially results in unburned hydrocarbon emissions. Further, these references do not address the challenge of dynamic venting where gaseous fuel is vented from the rail, either purposively or through leakage during operation of the engine.
The state of the art is lacking in techniques for reducing vented emissions from gaseous fuelled internal combustion engines. The present apparatus and method provides a technique for improving the handling of vented gaseous fuel from internal combustion engines.